1. Field of the invention
The invention relates to a method of making field emission tips using randomly located discrete nuclei deposited by physical vapor deposition as an etch mask.
2. Description of Related Art
Field emitters are widely used in ordinary and scanning electron microscopes since emission is affected by the adsorbed materials. Field emitters have also been found useful in flat panel displays and vacuum microelectronics applications. Cold cathode and field emission based flat panel displays have several advantages over other types of flat panel displays, including low power dissipation, high intensity and low projected cost.
The present invention can be better appreciated with an understanding of the related physics. General electron emission can be analogized to the ionization of a free atom. Prior to ionization, the energy of electrons in an atom is lower than electrons at rest in a vacuum. In order to ionize the atom, energy must be supplied to the electrons in the atom. That is, the atom fails to spontaneously emit electrons unless the electrons are provided with energy greater than or equal to the electrons at rest in the vacuum. Energy can be provided by numerous means, such as by heat or irradiation with light. When sufficient energy is imparted to the atom, ionization occurs and the atom releases one or more electrons.
Several types of electron emissions are known. Thermionic emission involves an electrically charged particle emitted by an incandescent substance (as in a vacuum tube or incandescent light bulb). Photoemission releases electrons from a material by means of energy supplied by incidence of radiation, especially light. Secondary emission occurs by bombardment of a substance with charged particles such as electrons or ions. Electron injection involves the emission from one solid to another. Finally, field emission refers to the emission of electrons due to an electric field.
In field emission (or cold emission), electrons under the influence of a strong electric field are liberated out of a substance (usually a metal or semiconductor) into a dielectric (usually a vacuum). The electrons "tunnel" through a potential barrier instead of escaping "over" it as in thermionics or photoemission. Field emission is therefore a quantum-mechanics phenomena with no classical analog. A detailed explanation of field emission quantum mechanics is beyond the scope of the discussion herein. Nevertheless, field emitters have been extensively studied and are well known in the art. See generally, P. H. Cutler and T. T. Tsong, Field Emission and Related Topics, 1978.
The shape of a field emitter effects its emission characteristics. Field emission is most easily obtained from sharply pointed needles or tips whose ends have been smoothed into a nearly hemispherical shape by heating. Tip radii as small as 100 nanometers have been reported. As an electric field is applied, the electric lines of force diverge radially from the tip and the emitted electron trajectories initially follow these lines of force. Devices with such sharp features similar to a "Spindt cathode" have been previously invented. An overview of vacuum electronics and Spindt type cathodes can be found in the November and December, 1989 issues of IEEE-Transactions of Electronic Devices. Fabrication of such fine tips, however, normally requires extensive fabrication facilities to finely tailor the emitter into a conical shape. It can be difficult to build high-performance large-area field emitters (i.e. more than a few sources per square micron area) since the cone size is limited by the lithographic equipment. It is also difficult to perform fine feature photolithography on large area substrates as required by flat panel display type applications. Thus, photolithography often significantly increases both the cost and complexity of the process.
The work function of the electron emitting surface or emission tip of a field emitter also affects emission characteristics. The work function is the voltage (or energy) required to extract or emit electrons from a surface. The lower the work function, the lower the voltage required to produce a particular amount of emission.
Prior attempts to fabricate field emitter devices have included the following:
U.S. Pat. No. 3,947,716 describes enhancing emission quality by altering the molecular structure of a tip to lower the work function of the planes where increased adsorption is desired, and increasing the work function in regions where less adsorption is desired. The molecular alteration is accomplished with thermal field buildup by subjecting the emitter tip to heating pulses in the presence of an electrostatic field to cause surface migration of the tip atoms along the field lines.
U.S. Pat. No. 3,970,887 describes a semiconductor substrate with integral field emitter tips in which an insulating layer and conductive layer are provided with openings at the tips to form micro-anode structures for each tip. The tips are electrically isolated from one another by initially doping the substrate to provide opposite conductivity-type regions at each tip.
U.S. Pat. No. 3,998,678 describes making field emitters by forming a metal layer on a substrate, forming conical emitters on the metal, forming an insulating layer with height equal to the emitters, forming another metal layer as an accelerating electrode, and etching he insulating layer to expose the emitters.
U.S. Pat. No. 4,307,507 describes making field emitters by selectively masking a single crystal substrate so that the unmasked regions define islands, orientation-dependent etching the substrate to from holes whose sides intersect at a crystallographically sharp point, removing the mask, covering the substrate and filling the holes with an emissive material, and etching the substrate to expose sharp emission tips.
U.S. Pat. No. 4,685,996 describes making field emitters by anisotropically etching a single crystal silicon substrate through photoresist openings to form funnel shaped protrusions thereon, then conformally depositing a refractory metal onto the protrusions.
U.S. Pat. No. 4,933,108 describes making field emitters by coating a metal carrier wire with crystals of an oxide compound which has a metallic luster and which is a compound transition metal selected from the group of tungsten, molybdenum, niobium, vanadium and titanium.
U.S. Pat. No. 4,943,343 describes a self-aligned gate process for making field emitters which includes forming conical emission tips on a substrate, depositing oxide on the tips, depositing gate metal on the oxide, depositing photoresist on the gate metal, etching the resist to expose gate metal above the tips, etching the exposed gate metal, removing the resist, and etching the exposed oxide to expose the emission tips.
A primary shortcoming and deficiency in much of the prior art is the inability to form fine conical or pyramid shaped features or emission tips without photolithography. Prior attempts to fabricate graded microtips without photolithography have included the following:
U.S. Pat. No. 4,465,551 discloses depositing a globular layer material onto a surface such that the material self-agglomerates into a layer of non-uniform thickness. The globular material may be formed as separate islands of randomly located etch mask material. The globular materials set forth are either polymerized monomers or metals of relatively low melting points such as aluminum evaporated onto a heated surface. An etch forms a graded-index layer which is useful in optical reflection reduction. It is often necessary to deposit an intermediate layer between the surface and the globular layer in order to assure proper globular layer formation. Moreover, a reactive etch is always used, that is, the etch is performed in the presence of a gas which reacts with the surface to accelerate the etch and increase the etch rate ratio. Although the basic theory of this method appears to be sound, in practice this method has a number of serious drawbacks. Reactive etching tends to be expensive and demands precise control over the flow of gas. For cost-effective manufacturing it is highly desirable to eliminate reactive gases. Furthermore, an appropriate combination of globular material and reactive gas must be selected to achieve a desired etch rate, which further limits the choice of globular materials.
U.S. Pat. No. 4,806,202 discloses a plasma etch process utilizing an oxide etcher with high energy ion bombardment and an aluminum electrode. Sputtered aluminum from the electrode occurs on the surface oxide and blocks some of the etching due to the anisotropic nature of the etch. As a result, grass like oxide residue is formed. A drawback, however, is that high energy ions are directed simultaneously to the oxide surface and the aluminum electrode. Thus, the oxide surface is subjected to etching before the aluminum is liberated from the aluminum electrode to form the etch mask. In addition, reactive etching is used, and sputtering aluminum is not generally scalable to depositing on large area substrates and tends to produce relatively non-uniform layers. Moreover, the grass like oxide residue lacks the pyramid type shape necessary for adequate field enhancement during field emission.
Therefore there still exists a need for relatively simple methods of making sub-micron pyramid shaped field emission tips of low work function materials without the use of photolithography.